Vehicle and method for managing power limits for a battery

ABSTRACT

A method for managing power limits for a battery includes the step of increasing a minimum operating state of charge after an initial power capability has decreased to the point where a predefined full discharge power is not available at an initial minimum operating state of charge. The increased minimum operating state of charge can be chosen such that the full discharge power is available. The increased minimum state of charge may not be chosen to provide the full discharge power if the increased minimum state of charge is greater than a maximum low limit state of charge.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/943,429 filed 10 Nov. 2010, which is hereby incorporated herein byreference.

BACKGROUND

1. Technical Field

The present invention relates to a method for managing power limits fora battery.

2. Background

A battery used in an electric vehicle or hybrid electric vehicle (HEV),like any battery, has a finite life. Specifically, the power capabilityof a battery decreases as the battery ages. HEV batteries are oftencontrolled such that they operate at or near a 50% state of charge(S.O.C.), with a full battery power being available over some range ofstates of charge. For example, a battery in an HEV may be controlledsuch that the S.O.C. is in the range of 40%-60%. When a battery operatesoutside its desired S.O.C. range, the power limit will be reduced sothat the battery is not asked to exceed its capability on either thecharge or discharge side.

As a battery ages, its power capability decreases, and the power limitsmust be accordingly reduced both inside and outside the desired S.O.C.operating range. Because the power limits can be represented by a curve,one way to reduce the power limits is to reduce the entire curve by somepower value to ensure that the entire curve remains under the powercapability curve at every S.O.C. This method may have the disadvantageof significantly reducing the power limits over the entire S.O.C.operating range, when in fact, it may be possible to have a higher powerlimit over at least some of the S.O.C. operating range.

Therefore, it would be desirable to have a control strategy for managingbattery power limits such that higher power limits are made availableeven after a battery ages and its power capability is reduced.

SUMMARY

Embodiments of the present invention include a method for managing powerlimits for a battery having a power capability that is a function of atleast the battery S.O.C. The battery has an operating S.O.C. rangedefined by a minimum operating S.O.C. and a maximum operating S.O.C. Italso has a discharge power limit that is a function of at least theS.O.C. An initial discharge power limit is set below an initial powercapability such that a predefined full discharge power is available atstates of charge above an initial minimum operating S.O.C. The methodincludes determining whether the full discharge power is available atthe initial minimum operating S.O.C. after the initial power capabilityhas decreased. A new S.O.C. at which the battery can supply the fulldischarge power is determined when it is determined that the fulldischarge power is not available at the initial minimum operating S.O.C.It is then determined whether the new S.O.C. is greater than a maximumlow limit S.O.C., and the minimum operating S.O.C. is set to the newS.O.C. if it is determined that the new S.O.C. is not greater than themaximum low limit S.O.C.

Embodiments of the invention also include a method for managing powerlimits for a battery that includes increasing a minimum operating S.O.C.after an initial power capability has decreased such that a predefinedfull discharge power is not available at an initial minimum operatingS.O.C. The increased minimum operating S.O.C. is chosen such that thefull discharge power is available if the increased minimum S.O.C. is notgreater than a maximum low limit S.O.C. Conversely, if the S.O.C. wherefull discharge power is available is greater than the maximum low limitS.O.C., and a power capability of the battery at the maximum low limitS.O.C. is not less than a predetermined power capability, then theincreased minimum operating S.O.C. is set to the maximum low limitS.O.C. The predetermined power capability can be defined as being equalto an operational minimum power capability plus a predetermined amount,such as a small buffer. In the case of an HEV, the operational minimumpower capability may be defined as the amount of power required to startthe engine.

The method may also include setting the increased minimum operatingS.O.C. to the maximum low limit S.O.C. if the S.O.C. where fulldischarge power is available is greater than the maximum low limitS.O.C., and a temperature of the battery is not within a predeterminedrange. An end of life for the battery may be indicated if the S.O.C.where full discharge power is available is greater than the maximum lowlimit S.O.C., the power capability at the maximum low limit S.O.C. isless than a predetermined power capability, and the temperature of thebattery is within the predetermined temperature range. The maximum lowlimit S.O.C. allowed during a fault condition of the battery may bedefined as a “default condition maximum low limit S.O.C.”. Similarly, anabsolute maximum low limit S.O.C. may be defined as the S.O.C. at whichthe battery can supply a minimum operational discharge power. A methodof the present invention may also include the step of setting themaximum low limit S.O.C. to the absolute maximum low limit S.O.C. if theabsolute low limit S.O.C. is not greater than the default conditionmaximum low limit S.O.C.

Embodiments of the invention also include a method for managing powerlimits for a battery that includes reducing a maximum operating S.O.C.after an initial power capability has decreased, such that a predefinedfull charge power is not available at an initial maximum operatingS.O.C. The reduced maximum operating S.O.C. is chosen such that the fullcharge power is available if the reduced maximum S.O.C. is not less thana minimum high limit S.O.C. Conversely, the reduced maximum operatingS.O.C. is set to the minimum high limit S.O.C. if it is determined thatthe reduced maximum operating S.O.C. is less than the minimum high limitS.O.C.

Embodiments of the invention include a method for managing power limitsfor a battery, including the steps of determining whether the fulldischarge power is available at the initial minimum operating S.O.C.after the initial power capability has decreased. The discharge powerlimit is reduced at a first predetermined S.O.C. to a first point belowa present power capability curve. A first portion of a discharge powerlimit curve is defined as containing the first point and being generallyparallel to the present power capability curve. A second portion of thedischarge power limit curve is defined as having a generally constantpower level above a second predetermined S.O.C.

The first predetermined S.O.C. may be, for example, the initial minimumoperating S.O.C. The second predetermined S.O.C. may be, for example,the present S.O.C. of the battery. Alternatively, the secondpredetermined S.O.C. could be defined as the point at which the firstportion of the discharge power limit curve intersects the initialdischarge power limit curve at a point above the initial minimumoperating S.O.C. The discharge power limit curve may also have a thirdportion defined as being coincident with a portion of the initialdischarge power limit curve below the initial minimum operating S.O.C.

Embodiments of the invention may also include a method for managingpower limits for a battery that include the steps of reducing adischarge power limit curve for the battery after a present powercapability curve has lowered such that a predefined full discharge poweris not available at an initial minimum operating S.O.C. The reduceddischarge power limit curve has a first portion that is generallyparallel to the present power capability curve, and a second portionthat is generally horizontal above a predetermined S.O.C. The secondportion of the reduced discharge power limit curve may begin at thepresent S.O.C., or alternatively, may begin at the point of intersectionbetween the reduced discharge power limit curve and the initialdischarge power limit curve above the initial minimum operating S.O.C.The reduced discharge power limit curve may include a third portion thatis coincident to an initial discharge power limit curve below theinitial minimum operating S.O.C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an HEV containing a batterycontrolled in accordance with embodiments of the present invention;

FIG. 2 is a graph showing a battery discharge power limit managed inaccordance with an embodiment of the present invention;

FIG. 3 is a graph showing a battery discharge power limit managed inaccordance with an embodiment of the present invention;

FIG. 4 is a graph showing a battery discharge power limit for a batterynearing its end of life managed in accordance with an embodiment of thepresent invention;

FIG. 5 is a graph showing a battery charge power limit managed inaccordance with an embodiment of the present invention;

FIG. 6 is a flow chart illustrating a method of managing a dischargepower limit for a battery in accordance with an embodiment of thepresent invention;

FIG. 7 is a flow chart illustrating a method of managing a dischargepower limit for a battery in accordance with an embodiment of thepresent invention;

FIG. 8 is a flow chart illustrating a method of managing charge powerlimits for a battery in accordance with an embodiment of the presentinvention; and

FIG. 9 is a graph showing a battery discharge power limit managed inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a vehicle 10 having an engine 12, an electric motor14, a battery 16, and a battery control module (BCM) 18. Although thevehicle 10 is an HEV, it is understood that battery control methods inaccordance with the present invention can be used to control batteriesthat power other types of systems and equipment, for example, electricvehicles having no engine, fuel cell electric vehicles, etc. Asdescribed in detail below, embodiments of the present invention includemethods for managing power limits for a battery, such as the battery 14.These methods may be part of a control strategy programmed into one ormore controllers, such as the BCM 18. It is understood that the BCM maybe part of a larger controller area network (CAN) that includes avehicle system controller (VSC), a powertrain control module (PCM), andone or more controllers dedicated to a particular piece of equipment,like the BCM 18.

FIG. 2 shows a graph 20 illustrating the power capability 22 of abattery, such as the battery 14 shown in FIG. 1. The graph 20illustrates the discharge power side of a battery whose power capability22 is reduced from an initial power capability the battery had when itwas new. An initial discharge power limit, shown by the curve 24 isabove the power capability curve 22 for certain states of charge. Aninitial power capability of the battery may have had a curve similar inshape to the curve 22; however, it would have been above the powerdischarge limit curve 24 for all states of charge.

In general, a discharge power limit, or on the other side of the curve acharge power limit, is chosen to be somewhere below the power capabilityof the battery at the time the power limits are set. It can be based ona number of factors, including the needs of the system being operated bythe battery, in the case of the vehicle 10 shown in FIG. 1, the powerrequirements of the motor 14 needed to start the engine 12 and/or propelthe vehicle 10, and may also be dependent on the physical constraints ofthe battery itself. As shown in FIG. 2, the discharge power limit 24 isgenerally constant at 35 kilowatts (kW) above an S.O.C. of 36%. As notedabove, the initial power capability of the battery was above the initialdischarge power curve 24 for all states of charge. Thus, when thebattery was new, the highest portion of the curve 24, which is at 35 kW,represented the full discharge power for the battery. The full dischargepower was available starting at an S.O.C. of about 36%, which representsa minimum operating S.O.C. for the battery. As shown in FIG. 2, thepower capability 22 of the battery is below the initial discharge powerlimit 24 at the minimum operating S.O.C. of 36%. As a result, it isnecessary to reduce the discharge power limit for the battery so that itis once again below the power capability 22.

One way to reduce the discharge power limit so that it is below thepower capability 22 is to reduce the power discharge limit curve 24 overthe S.O.C. operating range by some predetermined amount—this isillustrated by the curve 26. Reducing the discharge power limit in thisway does ensure that the discharge power limit 26 is below the powercapability 22 at the initial minimum operating S.O.C. of 36%; however,it undesirably keeps the discharge power limit lower than it needs to beover much of the S.O.C. operating range. This is where the controlstrategy of the present invention can be employed to more efficientlyutilize battery power.

Embodiments of the present invention examine the battery powercapability at the initial minimum operating S.O.C. after the batterycapability has been reduced from its initial state. Using FIG. 2 forreference, a method of the present invention would determine whether thefull discharge power of 35 kW is available at the initial minimumoperating S.O.C. of 36% after the initial power capability of thebattery has decreased. As illustrated in FIG. 2, the reduced powercapability 22 of the battery provides only about 27.5 kW discharge powerat an S.O.C. of 36%. Rather than reducing the discharge power limitcurve 24, such as illustrated by the reduced curve 26, embodiments ofthe present invention determine a new S.O.C. at which the battery cansupply the full discharge power of 35 kW even given the reduced powercapability curve 22. As shown in FIG. 2, this new S.O.C. is a littleunder 44%.

Because it is not desirable to have the minimum operating S.O.C. to betoo great, it may be desirable to provide a maximum low limit S.O.C. tobe the upper boundary of the minimum operating S.O.C. Again using FIG. 2for reference, if the maximum low limit S.O.C. was above the new S.O.C.of approximately 44%, then the minimum operating S.O.C. would be changedfrom its initial value of 36% to the new S.O.C. of approximately 44%.If, however, the maximum low limit S.O.C. was below 44%, embodiments ofthe present invention would not set the minimum operating S.O.C. at thislevel.

In the example shown in FIG. 2, the maximum low limit S.O.C. is 40%;therefore, even though the full discharge power is not available untilafter an S.O.C. of 44%, the minimum operating S.O.C. will be changedfrom 36% only to 40%. As will be discussed in detail below, thedischarge power limits are often changed along with the charge powerlimits and the combination of these changes can lead to a narrowing inthe S.O.C. operating range. Because it is undesirable to have too smallof an operating range, the minimum operating S.O.C. will not beincreased beyond the maximum low limit S.O.C.

Thus, the curve 28 shown in FIG. 2 represents the new discharge powerlimit set in accordance with an embodiment of the present invention.Although the maximum discharge power is less than the full dischargepower of 35 kW that was available with the new battery, it is well abovethe maximum discharge power of 27 kW that would have been set using thebasic offset method shown by the curve 26. It is worth noting that ifthe power capability 22, at the maximum low limit S.O.C. of 40%, wasbelow some predetermined power capability, the battery may have beenmarked as end of life. This is described in more detail below. In theexample shown in FIG. 2, however, the power capability 22 of the batterywas not so low as to require end of life, and a new discharge powerlimit curve 28 could be implemented so as to continue to use the batteryat the relatively high level of 31 kW over the new S.O.C. operatingrange.

Because the temperature of the battery can also have an effect on itscharge and discharge power capability, embodiments of the presentinvention may also make a determination of whether a temperature of thebattery is within a predetermined range. If it is not within thepredetermined range, but the power capability of the battery is belowthe predetermined power capability, the battery may not be marked forend of life because the low battery power capability may be only atemporary result of the out of range temperature. In such a case, theminimum operating S.O.C. will still be moved to the maximum low limitS.O.C., which is 40% in the example shown in FIG. 2.

As shown in FIG. 2, the discharge power limit curve 28 slopes down tozero as it goes below the maximum low limit S.O.C. of 40%. Although thecurve 28 and the power capability curve 22 are very close at an S.O.C.of 40%, the two curves quickly diverge below 40% such that power thatmight otherwise be available to a vehicle operator is artificiallylimited by the slope of the discharge power limit curve 28. Embodimentsof the present invention address this issue, and this is illustrated inthe graph 30 shown in FIG. 3. In FIG. 3, the curves 22′, 24′, 26′ areall analogous to the curves 22, 24, 26 shown in FIG. 2. The curve 32,however, is different from the curve 28 once the S.O.C. drops below 40%.Above 40%, the curve 32 is, like the curve 28 shown in FIG. 2, constantat 31 kW. Below an S.O.C. of 40%, however, the curve 32 does not slopedirectly to zero as the curve 28 does in FIG. 2. Rather, the curve 32more closely resembles the slope of the curve 22′, thereby maintainingadditional power availability for a vehicle operator at lower states ofcharge.

One way to determine the slope of the curve 32 below an S.O.C. of 40% isto define a first point A as the discharge power limit at the minimumoperating S.O.C. of 40%. Next, a second point is defined at the initialminimum operating S.O.C. (recalling from FIG. 2 this value is 36%),where the second point is some distance below the power capability. Thisis illustrated as point B in FIG. 3. A first portion of the dischargepower limit curve 32 below the minimum operating S.O.C. of 40% is thendefined by the line connecting point A and point B. A second portion ofthe discharge power limit curve 32 below the minimum operating S.O.C. of40% can then be defined by a line segment 34 that has a slope thatgenerally matches the slope of the power capability curve 22′ and alsocontains the point B. After point C, the slope of the discharge powerlimit 32 is run to zero, in this case along the same line as the initialdischarge power limit 24′.

The discussion above can be represented in equation form as follows:

${DPL}_{raw} = {{DPL}_{a} - {( {{DPL}_{a} - {DPL}_{b}} )^{\prime}\frac{( {{SOC} - {SOC}_{a}} )}{( {{SOC}_{a} - {SOC}_{b}} )}}}$where:

SOCa and DPLa are the S.O.C. and discharge power limit (capability lessany desired buffer) at Point A,

SOCb and DPLb are the S.O.C. and discharge power limit (capability lessany desired buffer) at Point B, and

SOC is the actual state of charge of the battery.

A similar equation can be used for management of a charge power limit,such as the charge power limit discussed below in conjunction with FIG.5. For example:

${CPL}_{raw} = {{CPL}_{a} - {( {{CPL}_{a} - {CPL}_{b}} )^{\prime}\frac{( {{SOC}_{a} - {SOC}} )}{( {{SOC}_{a} - {SOC}_{b}} )}}}$where:

SOCa and CPLa are the S.O.C. and charge power limit (capability less anydesired buffer) at Point A,

SOCb and CPLb are the S.O.C. and charge power limit (capability less anydesired buffer) at Point B, and

SOC is the actual state of charge of the battery.

FIG. 4 shows a graph 36 showing the power capability 38 of a batterynearing end of life. The curve 40 represents the initial discharge powerlimit set when the battery was new. As clearly shown in FIG. 4, thepower capability 38 is now well below the initial discharge power limit40, which itself was set below the initial power capability of thebattery. As noted above, when the power capability of a battery isreduced, the power discharge limits are also reduced to ensure that thedemand on the battery does not exceed its capability. One way to do thiswould be to set the discharge power limit in accordance with the curve42, which provides a minimum operating S.O.C. at or near the maximum lowlimit S.O.C. of 40%. Although this keeps the discharge power limit belowthe power capability 38, it unnecessarily limits the amount of poweravailable to the vehicle operator because of the limitation on themaximum low limit S.O.C. of 40%. To address this issue, embodiments ofthe present invention allow the minimum operating S.O.C. to go beyondthe maximum low limit S.O.C. when the battery is near end of life.

A control strategy in accordance with the present invention, may, forexample, set the battery discharge power limit for a near end of lifebattery in accordance with the curve 44 shown in FIG. 4. In this case,the minimum operating S.O.C. is approximately 49%, which is well abovethe maximum low limit S.O.C. of 40% discussed above. This exception ismade, however, because the battery is near its end of life, andincreasing the discharge power limit will allow the battery to be usedfor a longer time without being marked as end of life. As discussedabove, embodiments of the present invention may determine whether areduced power capability of the battery is below some predeterminedpower capability. This predetermined power capability may be a minimumoperational power capability plus some small amount such as a buffer.When it is determined that the power capability of the battery hasdropped below this level, the end of life strategy can be implemented.For example, until this time the minimum operating S.O.C. may be limitedto the maximum low limit S.O.C. In the examples used above this was 40%.Upon determination that the power capability of the battery has droppedbelow the predetermined power capability, the minimum operating S.O.C.may be raised above this level.

In general, the minimum operating S.O.C. will be raised to a point wherethe battery has the desired power. In the case of an HEV, the desiredpower will be enough to start the vehicle engine so as to not strand thevehicle operator. The minimum operating S.O.C. will not, however, be setso high that the battery will be overcharged if it is to accept someminimum level of charge, for example, through regenerative breaking. Oneformula by which the new increased minimum operating S.O.C. can bedetermined is to take the amount of power that the battery needs toaccept for regenerative breaking and then determine the S.O.C. at whichthis power could be accepted.

In one example, the battery may be required to accept 5 kW of power forregenerative breaking, and it could accept this amount of power when theS.O.C. is at 90%. In addition to this criteria, two other parameters maybe evaluated. First, an S.O.C. imbalance in the battery may bedetermined to account for S.O.C. variations in the individual cellsmaking up the battery. In addition, some minimum operating S.O.C. rangewill still need to be maintained. Continuing with the example startedabove, if the S.O.C. imbalance in the battery is 5% and the minimumallowable size of the S.O.C. operating range is 5%, then embodiments ofthe present invention subtract from the predetermined 90% both the 5%S.O.C. imbalance and the 5% minimum allowable size of the S.O.C.operating range to get a result of 80% S.O.C. This 80% S.O.C. thenrepresents the highest level to which the minimum operating S.O.C. canbe raised when the power capability of the battery has fallen below thepredetermined power capability amount.

Although the examples thus far have focused on managing the dischargepower limit for a battery, similar techniques can be employed withregard to a charge power limit on the high side of the S.O.C. for thebattery. In FIG. 5 is a graph 46 showing the application of anembodiment of the present invention to a charge power limit for abattery. The curve 48 shows the power capability for the battery afterit has been reduced from some initial power capability. The curve 50shows the initial power limit which is now above the power capability 48for certain ranges of the S.O.C. When the initial charge power limit 50was determined, a full charge power was set at 35 kW. The highest S.O.C.at which the full charge power was available was 64%; after the S.O.C.of 64%, the initial charge power limit 50 is reduced to zero.

One way to manage charge power limits for a battery after the powercapability of the battery has degraded, is to reduce the initial chargepower limit by some predetermined amount. This is illustrated by thecurve 52, where the maximum operating S.O.C. is still set at 64%. Asshown in the graph 46, the power level of the curve 52 is just below thepower capability 48 at an S.O.C. of 64%. Thus, the amount of poweravailable to a vehicle operator has been reduced from the full chargepower of 35 kW to a level of just over 26 kW.

Application of the present invention to the charge power managementprovides an increase in available power to the vehicle operator over awide range of S.O.C.; this is shown by the curve 54. Just as with themanagement of the discharge power limit, a method of the presentinvention may start by determining an S.O.C. at which the full chargepower of 35 kW is available even with the reduced power capabilityindicated by the curve 48. As shown in FIG. 5, this S.O.C. is just under57%. Thus, if the maximum operating S.O.C. were reduced to 57%, the fullcharge power of 35 kW would be available to the vehicle operator atS.O.C.'s below 57%. Just as the minimum operating S.O.C. was limited bysome maximum low limit S.O.C., so too is the maximum operating S.O.C.limited by some minimum high limit S.O.C., which may be convenientlyreferred to as the “minimum high limit S.O.C.”. In general, as theminimum operating S.O.C. is increased to provide full discharge power,and the maximum operating S.O.C. is decreased to provide full chargepower, the operating range of the S.O.C. is decreased.

In the example shown in FIG. 5, the minimum high limit S.O.C. has beenset at 60%. Thus, when it is determined that the full charge power of 35kW is available only if the maximum operating S.O.C. is reduced to 57%,the control strategy of the present invention sets a lower boundary forthe maximum operating S.O.C. at 60%. This is illustrated by the curve54, which is just below the power capability 48 at an S.O.C. of 60%.Although this management strategy results in the lowering of the chargepower available to the operator from a full charge power of 35 kW to acharge power of approximately 31 kW, it provides a significant advantageover the method illustrated by curve 52, which would provide a maximumcharge power for the vehicle operator of only 26 kW. In addition, theremay be many times in which the battery power capability has beenreduced, and the reduction in maximum operating S.O.C. to provide fullcharge power will still be higher than the minimum high limit S.O.C.,which means that the vehicle operator will still have the full chargepower available, although the S.O.C. operating range will be somewhatreduced.

Turning now to FIGS. 6-9, methods in accordance with embodiments of thepresent invention are illustrated and described in a number of flowcharts. The flow chart 56, shown in FIG. 6, starts with step 58 wherethe discharge power capability of the battery is determined. At decisionblock 60, it is determined whether the discharge power capability at theminimum operating S.O.C. is less than the discharge power limit. Turningto FIG. 2, for example, step 60 determines whether the curve 22 is lowerthan the curve 24 at an S.O.C. of 36%. If the answer was “no”, themethod moves to step 62, where the discharge power limit 24 and theminimum operating S.O.C. of 36% is left unchanged.

This is not the case, however, in the example shown in FIG. 2. Rather,the curve 22 is below the curve 24 at an S.O.C. of 36%. Therefore, themethod moves to step 64, where it is determined the S.O.C. at which thebattery can supply the existing full discharge power—this point isapproximately 44% on the graph 20 shown in FIG. 2. The next inquiry atstep 66 is whether this determined value of S.O.C. is greater than themaximum low limit S.O.C. If it is not, the method moves to step 68 wherethe minimum operating S.O.C. is set to the S.O.C. determined at step 64.In the example shown in FIG. 2, however, this is not the case, and thedetermined S.O.C. of approximately 44% is greater than the maximum lowlimit S.O.C. of 40%. In this case, the method moves to step 70 where twoinquiries are made.

The first inquiry made in step 70 is whether the power capability of thebattery at the maximum low limit S.O.C. (40% in the example shown inFIG. 2) is less than some operational minimum plus a small bufferamount. The second inquiry is whether the temperature of the battery iswithin some predetermined range. If the answer to either of theseinquiries is “no” the method moves to step 72 where the minimumoperating S.O.C. is set to the maximum low limit S.O.C.—this is thesituation shown in FIG. 2. The temperature of the battery is analyzed,because a battery outside its normal operating range may have atemporarily reduced power capability that would return to within normallimits when the temperature of the battery returned to the normaloperating range.

If the answer to both of the inquiries at step 70 is “yes”, an end oflife is signalled for the battery—see step 74. A determination is thenmade as to the S.O.C. at which the battery can supply the minimumoperational discharge power—see step 76. At step 78, the inquiry is madeas to whether the S.O.C. determined in step 76 is greater than thehighest value the minimum operating S.O.C. can be moved to in a faultcondition—this was described in conjunction with FIG. 4 above. If theanswer is “no”, the minimum operating S.O.C. is set to the lowest S.O.C.that can still provide the minimum required discharge power; this isshown at step 80. If, however, the S.O.C. determined in step 76 isgreater than the highest value that it can be moved to, then the minimumoperating S.O.C. is set to the highest allowed value—see step 82.

As discussed above in conjunction with FIG. 3, embodiments of thepresent invention also provide for setting a discharge (or charge) powerlimit curve that more closely follows the battery capability curve; onesuch method is described in the flow chart 84 shown in FIG. 7. At step86 it is determined whether the current S.O.C. for the battery is abovethe minimum S.O.C. at which full discharge power is available. UsingFIG. 2 for an example, the inquiry at step 86 is whether the currentS.O.C. for the battery is above approximately 44%. If it is, there is noneed to adjust the discharge power limits, and this is illustrated atstep 88. If, however, the current S.O.C. is below this level, the methodmoves to step 90.

At step 90 a determination is made as to whether the S.O.C. at whichfull discharge power is available has been raised from its initialvalue. Turning to FIG. 3 to further the example, the answer to theinquiry at step 90 is “yes”. Point A, corresponding to an S.O.C. ofapproximately 40%, has been raised from the initial minimum operatingS.O.C. of 36%. If this were not the case, the method would move to step92, where the discharge power limit would be reduced linearly with theS.O.C. to some desired zero point. This is illustrated in FIG. 3 by thecurve 26′, where Point B was not raised from the initial minimumoperating S.O.C. of 36%, and the discharge power limit is reduced fromapproximately 26 kW, at an S.O.C. of 36%, to zero at an S.O.C. ofapproximately 28%.

In the example shown in FIG. 3, however, the answer to the inquiry instep 90 is “yes”; therefore, the method moves to step 94. At step 94,the point B is defined. At step 96, point A is defined, and a powerlimit curve is calculated by determining point C at step 98. At step100, the inquiry is made as to whether the point determined at step 98is greater than the discharge power limit for a new battery at the sameS.O.C. If the answer is “no”, the method moves to step 102, where thedischarge power limit is set at Point C. If, however, the answer to theinquiry at step 100 is “yes”, the method moves to step 104 where thedischarge power limit is set to what the discharge power limit is for anew battery at that S.O.C.

Turning to FIG. 8, the flow chart 106 shows a method analogous to theone illustrated in FIG. 6; however, the flow chart 106 shows a method ofthe present invention as applied to the charge power limit of a battery.At step 108 the power capability for the battery is determined; this isillustrated, for example, by the curve 48 in FIG. 5. At step 110 it isdetermined whether the power capability at the maximum operating S.O.C.(64% in the example shown in FIG. 5) is less than the charge power limit(curve 50 in the example shown in FIG. 5). If the answer is “no”, themethod moves to step 112 and the charge power limit is left unchanged.If, however, the answer to the inquiry at step 110 is “yes” then themethod moves to step 114.

At step 114 a determination is made as to the S.O.C. where full chargepower is available. Using the example in FIG. 5, this S.O.C. isapproximately 57%. As shown at step 116, this S.O.C. may need to beadjusted if it is higher than a maximum low limit S.O.C.; this inquiryis made at step 118. In the example shown in FIG. 5, the answer to thisinquiry was “yes”, in that the maximum low limit S.O.C. was 60%. In thiscase, the method moves to step 120 where the maximum operating S.O.C. isset to the maximum low limit S.O.C. If the answer to the inquiry in step118 had been “no”, the method would have moved to step 122 where themaximum operating S.O.C. would have been set to the S.O.C. determined instep 114.

FIG. 9 shows a graph 124 showing management of a battery discharge powerlimit in accordance with an embodiment of the present invention.Specifically, the curve 126 shows the power capability of a new battery,while the curve 128 shows the initial power discharge limit curve setwhen the power capability was in accordance with the curve 126. The fulldischarge power 129 is indicated by the horizontal portion of the curve128, which is reduced to zero starting at some initial minimum operatingS.O.C. indicated by S.O.C.₁. After some period of operating time, thepower capability of the battery is reduced, as indicated by the curve130. At this point it is clear that the full discharge power is notavailable at the initial minimum operating S.O.C.—i.e., at S.O.C.₁ thepower capability curve 130 is below the power discharge curve 128. Toadjust for this change in power capability, embodiments of the presentinvention may use the following method.

Initially, the power discharge limit is reduced from its initial leveldefined by the curve 128. This reduction can take place at a firstpredetermined point, such as the initial minimum operating S.O.C.(S.O.C.₁). As a starting point, the discharge power limit curve may bereduced to a point 132 just below the present power capability curve130. The point 132 may be set at some predetermined distance below thecurve 130, which may include a built-in buffer to account forinaccuracies in determining the current power capability. The reduceddischarge power limit may now be partially defined by a first portion134 that contains the first point 132 and is generally parallel to thepower capability curve 130.

A second portion of the reduced discharge power limit curve can then bedefined, and will generally be horizontal, indicating a constantdischarge power limit over this portion. The starting point for thesecond portion of the reduced discharge power limit curve can beindicated by a present S.O.C., shown in FIG. 9 as S.O.C.₂, and indicatedby the point 136. As shown in FIG. 9, beginning the second portion 138of the reduced discharge power limit curve at the present S.O.C. maystill provide a maximum discharge power limit that is less than theinitial full discharge power limit 129, but has advantages over previouspower management methods. For example, in previous power managementmethods the discharge power curve may be indicated by the horizontalline 140, which merely reduced the initial full discharge power 129 to apoint below the reduced power capability 130. The gain in availablepower achieved by the present invention as described above, is indicatedby the shaded region 142.

Another way to determine the second portion of the reduced powerdischarge curve, is to allow the first portion 134 of the reduced powerdischarge limit curve to continue parallel to the reduced powercapability curve 130 until it reaches a point of intersection with theinitial power discharge limit curve 129—this is indicated by point 144,corresponding to S.O.C.₃ in FIG. 9. Using this method, the fulldischarge power 129 is again available to the vehicle operator althougha reduction in available power occurs at an earlier S.O.C.—that is, itbegins to reduce at S.O.C.₃ rather than S.O.C.₂. Using this method, thesecond portion of the reduced power discharge limit curve is indicatedby the horizontal line 146, which is that portion of the initial powerdischarge limit curve 128 that is above the point 144 corresponding toS.O.C.₃. This results in an additional gain of available power,indicated by the shaded region 147.

In addition to the first and second portions 134, 138 (or 146), thereduced power discharge limit curve may also include a third portionhaving an S.O.C. range that is below the first and second portions. Asindicated by the sloped line 148, a prior method of reducing the powerdischarge limit curve may have started at the point 132 and reduced thecurve to zero at the same point that the initial discharge power limitcurve 128 reached zero. Applying a method of the present invention, athird portion 150 of the reduced discharge power limit curve iscoincident with the initial discharge power limit curve 128 below thepoint of intersection of the first portion 134 of the reduced dischargepower limit curve and the initial discharge power limit curve 128—thispoint is indicated at 152.

Thus, in at least some embodiments, the reduced discharge power limitcurve intersects the initial discharge power limit curve 128 in twoplaces at points 144, 152, where point 144 is the intersection pointabove the initial minimum operating S.O.C. (S.O.C.₁), and the point 152is the intersection of the curves below S.O.C.₁. In general, the reduceddischarge power limit curve may be defined by three portions, a firstportion 134 having an S.O.C. range below a second portion 138 (or 146),and a third portion 150 having an S.O.C. range below both the first andsecond portions. By keeping at least the first portion 134 of thereduced discharge power limit curve generally parallel to the powercapability 130, greater power is available to the vehicle operator thanmight otherwise be available through other methods of battery powermanagement. The method described above in accordance with FIG. 9 can beapplied to the charge limit as well as the discharge limit. Moreover, anend of life strategy, such as described above with reference to FIG. 4,can also be applied to a power management method described andillustrated in FIG. 9.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

1. A method for managing power limits for a battery having a powercapability that is a function of at least a state of charge (S.O.C.) ofthe battery, the battery further having a minimum operating S.O.C. and adischarge power limit that is a function of at least the S.O.C., aninitial discharge power limit being set below an initial powercapability such that a predefined full discharge power is available atstates of charge above an initial minimum operating S.O.C., the methodcomprising: determining whether the full discharge power is available atthe initial minimum operating S.O.C. after the power capability hasdecreased from the initial power capability; reducing the dischargepower limit at a first predetermined S.O.C. to a first point below apresent power capability curve; defining a first portion of a dischargepower limit curve as containing the first point and being generallyparallel to the present power capability curve over an S.O.C. range; anddefining a second portion of the discharge power limit curve as having agenerally constant power level above a second predetermined S.O.C. 2.The method of claim 1, wherein the first predetermined S.O.C. is theinitial minimum operating S.O.C.
 3. The method of claim 1, wherein thesecond predetermined S.O.C. is a present S.O.C. of the battery.
 4. Themethod of claim 1, wherein the initial discharge power limit is definedby an initial discharge power limit curve, and the second predeterminedS.O.C. is defined by an intersection of the initial discharge powerlimit curve and the first portion of the discharge power limit curve atan S.O.C. above the initial minimum operating S.O.C.
 5. The method ofclaim 4, wherein the first portion of the discharge power limit curvefurther intersects the initial discharge power limit curve at an S.O.C.below the initial minimum operating S.O.C.
 6. The method of claim 1,wherein the second portion of the discharge power limit curve has anS.O.C. range above an S.O.C. range of the first portion of the dischargepower limit curve.
 7. The method of claim 1, wherein the initialdischarge power limit is defined by an initial discharge power limitcurve, the method further comprising defining a third portion of thedischarge power limit curve as coincident with a portion of the initialdischarge power limit curve below the initial minimum operating S.O.C.8. A method for managing power limits for a battery, comprising:reducing a discharge power limit curve for the battery after a presentpower capability curve has lowered such that a predefined full dischargepower is not available at an initial minimum operating S.O.C., thereduced discharge power limit curve having a first portion generallyparallel to a present power capability curve over an S.O.C. range, and asecond portion generally horizontal above a predetermined S.O.C.
 9. Themethod of claim 8, wherein the predetermined S.O.C. is a present S.O.C.of the battery.
 10. The method of claim 8, wherein the predeterminedS.O.C. is defined by the intersection of the first portion of thereduced discharge power limit curve and an initial discharge power limitcurve, the intersection being at an S.O.C. above the initial minimumoperating S.O.C.
 11. The method of claim 8, wherein the first portion ofthe reduced discharge power limit curve intersects an initial dischargepower limit curve at an S.O.C. below the initial minimum operatingS.O.C.
 12. The method of claim 8, wherein the reduced discharge powerlimit curve includes a third portion coincident with an initialdischarge power limit curve at an S.O.C. below the initial minimumoperating S.O.C.
 13. The method of claim 12, wherein the second portionof the reduced discharge power limit curve has an S.O.C. range above theS.O.C range of the first portion of the reduced discharge power limitcurve, and the third portion of the reduced discharge power limit curvehas an S.O.C. range below the S.O.C. ranges of the first and secondportions of the reduced discharge power limit curve.
 14. A vehicle,comprising: an electric motor operable to propel the vehicle; a batteryoperable to provide electric power to the motor and having a powercapability that is a function of at least a S.O.C. of the battery, thebattery further having a minimum operating S.O.C. and a discharge powerlimit that is a function of at least the S.O.C., an initial dischargepower limit being set below an initial power capability such that apredefined full discharge power is available at states of charge abovean initial minimum operating S.O.C.; and at least one controllerconfigured to execute a control strategy including the steps of:determining whether the full discharge power is available at the initialminimum operating S.O.C. after the power capability has decreased fromthe initial power capability, reducing the discharge power limit at afirst predetermined S.O.C. to a first point below a present powercapability curve, defining a first portion of a discharge power limitcurve as containing the first point and being generally parallel to thepresent power capability curve over an S.O.C. range, and defining asecond portion of the discharge power limit curve as having a generallyconstant power level above a second predetermined S.O.C.
 15. The vehicleof claim 14, further comprising an engine operatively connected to themotor.
 16. The vehicle of claim 14, wherein the first predeterminedS.O.C. is the initial minimum operating S.O.C.
 17. The vehicle of claim14, wherein the second predetermined S.O.C. is a present S.O.C. of thebattery.
 18. The vehicle of claim 14, wherein the initial dischargepower limit is defined by an initial discharge power limit curve, andthe second predetermined S.O.C. is defined by an intersection of theinitial discharge power limit curve and the first portion of thedischarge power limit curve at an S.O.C. above the initial minimumoperating S.O.C.
 19. The vehicle of claim 18, wherein the first portionof the discharge power limit curve further intersects the initialdischarge power limit curve at an S.O.C. below the initial minimumoperating S.O.C.
 20. The vehicle of claim 14, wherein the initialdischarge power limit is defined by an initial discharge power limitcurve, the control strategy further including the step of defining athird portion of the discharge power limit curve as coincident with aportion of the initial discharge power limit curve below the initialminimum operating S.O.C.